Method and apparatus for producing carbon foam boards

Abstract

To provide a method for producing fine-pored carbon foam boards having a mainly homogeneous structure regarding a method for producing carbon foam boards from starch for use as an insulation material or a filter.SOLUTION: The problem is solved by sprinkling at least one very thin layer of foamable starch powder 4' onto a temperature-resistant boardlike substrate 4, introducing the starch-powder-coated substrate into a furnace, introducing air, argon or nitrogen into the furnace until an internal pressure of about 720 mbar to about 1000 mbar has been established, heating the furnace 2 to a foaming temperature of 180°C to 450°C to foam the starch powder, maintaining the temperature over a prolonged duration of up to 10 hours to stabilize the foamed starch into a foam board 11.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing carbon foam sheets from starch powder or from other foamable polymers / polymer mixtures in powder form for use as insulation material, filters or building materials. [Background technology]

[0002] Starch flours are generally composed of various polymers and proteins, or polymer mixtures, such as corn starch or other plant-based polymers. This may also include starch flours obtained from oat or cereal products. Starch is a naturally occurring biopolymer as a polysaccharide.

[0003] It is known that polymer-containing starch can be foamed under certain conditions when heated to a relatively high temperature, with the size of the resulting bubbles depending on the specific temperature. During this process, some of the polymer melts, while other parts of the polymer or other polymers decompose to form gas, which is, as it were, trapped in the molten polymer to form the desired foam as an intermediate product. The foam is brown in color and has a soft consistency.

[0004] However, if too high a temperature or unsuitable environmental conditions are chosen, the entire starch decomposes and no foam is produced; at best, a carbonized residue is produced. DISCLOSURE OF THEINVENTION [Problem to be solved by the invention]

[0005] The present invention is based on the problem of providing a method and an apparatus for producing microporous carbon foam sheets with a largely uniform structure, which ensures uniform foaming of foamable starch powder, polymer or polymer mixtures and which functions reliably without the need for additional water vapor.

[0006] The term "thin layer" (or "thin layer") in the following should be understood as a coating which completely and uniformly covers the surface of the substrate as an essentially closed layer and in which the individual particles of the starch powder are mainly adjacent to one another. [Means for solving the problem]

[0007] The problem on which the present invention is based is - spreading at least one layer as thin as possible of foamable starch powder or of another foamable polymer or polymer mixture in powder form onto a heat-resistant sheet-like base, - placing said base coated with starch powder in a furnace; - introducing argon or nitrogen into the furnace by replacing the air or by pumping the air out of the high-temperature furnace until an internal pressure of about 700 to about 1,000 mbar is adjusted; - heating the oven to a foaming temperature of 180°C up to 450°C to foam the starch powder on said base; - maintaining said foaming temperature in the oven for an extended period of time up to 10 hours to stabilize said foamed starch into a foamed sheet; - carbonizing the stabilized foam sheet in a suitable furnace at >1,000°C under the exclusion of air in an argon or nitrogen atmosphere; This is resolved by:

[0008] The foam sheet carbonized in the furnace can then be graphitized in the same furnace at temperatures >2,000° C. under protective gas or in vacuum, said furnace being capable of being a vacuum high-temperature furnace.

[0009] As the foamable starch, powdered corn starch can be preferably used, since it can be foamed particularly well and uniformly. Alternatively, other polymers or polymer mixtures in powder form, such as tapioca starch, wheat starch, etc., can also be used for foaming.

[0010] Heating to the foaming temperature is preferably carried out at a heating rate of about 5° C. / min.

[0011] The sheet substrate may be made of a graphitizable material that will shrink to the same extent along with the foam sheet during carbonization and optional graphitization.

[0012] Alternatively, the sheet substrate may be made of steel, graphite or other heat resistant material, in which case the stabilized foam sheet must be peeled off the sheet substrate prior to carbonization.

[0013] The expandable starch powder is screened prior to or during spreading onto the sheet substrate to ensure that the spread layer has a uniform particle size.

[0014] To produce thicker foamed sheets, after each foaming or stabilization, another, as thin as possible, layer of starch powder is spread onto the already existing thinner foamed layer and the foaming is repeated.

[0015] In principle, the foaming temperature can also be lowered slightly so that a thinner but more solid foam layer can result, although in this case the foaming of the starch can take place over a longer period of time.

[0016] If foaming is carried out under air, the foaming temperature should be a maximum of 250°C.

[0017] If the starch powder is applied locally in different layer thicknesses onto a sheet-like base or onto an already foamed foam sheet, a structured surface of the foam sheet can be achieved.

[0018] When applying a relatively thick layer of starch powder, it is important and necessary to reduce the foaming temperature by about 10%, which requires an increase in the time for which the foaming temperature must be kept constant in order to facilitate the diffusion of the gas generated during foaming from such a relatively thick layer and to achieve uniform foaming. Alternatively, "fresh" starch can be periodically sprinkled onto the foamed starch.

[0019] To seal the surface, the starch powder spread on the sheet base can be coated with a powdered polymer.

[0020] The heat-resistant sheet substrate can consist of a graphitizable material, e.g. a polymer, or it can consist of steel, graphite, or a material with sufficient heat resistance, i.e., >250° C. In the latter case, the stabilized foam sheet must be removed from the sheet substrate before graphitization.

[0021] If a graphitizable material is used as the sheet-like substrate, this can be graphitized together with the foam top layer. Paper is a suitable graphitizable material.

[0022] In summary, the method for producing a carbon foam sheet comprises the following steps: - spreading the starch / polymer or polymer mixture onto a substrate; - placing the coated substrate in a furnace and increasing the temperature to a foaming temperature of 180°C to 450°C, Foaming is carried out in air or under an argon or nitrogen atmosphere, ○ The foaming is carried out under an internal pressure in the furnace of about 700 mbar to about 1,000 mbar; ○ The foaming temperature influences the density of the foamed sheet, i.e. the higher the density, the stronger the foam but the less insulating it will be; - repeating the steps to produce a thicker foam sheet; Includes.

[0023] The foam sheet thus produced has a brownish color.

[0024] The apparatus for carrying out this method consists of a space or box filled with argon or nitrogen, in which there is a furnace with a heating zone in which the sheet-like substrate coated with starch powder is placed.

[0025] The oven can be, for example, a continuous oven, which comprises a conveying device for conveying the starch powder coated substrate from a feed position to a heating zone and from there to a removal position, where said feed position and removal position can be spatially co-located or adjacent to one another.

[0026] The furnace may be operated at a temperature up to about 450° C. and a negative pressure ranging from about 700 mbar to about 1,000 mbar.

[0027] The transport device is preferably a high temperature resistant conveyor belt.

[0028] Furthermore, above the feed position there is arranged a container connected to the shaking device, in which the starch powder to be spread on the base is located.

[0029] Furthermore, the container is equipped with a sieving device to ensure uniform particle size of the starch powder spread on the base.

[0030] In an alternative method, for producing carbon foam sheets from starch powder or other foamable polymers / polymer mixtures in powder form, a heatable vacuum high-temperature furnace is provided, which is equipped with an evacuable, insulated housing, which is provided with a gas inlet for inert gas protruding into the housing from the side, where the gas inlet is simultaneously connected to a supply device for starch powder, and where in the housing, below the gas inlet, there is a place for depositing the starch powder in layers.

[0031] Starch powder fed to the gas inlet via a feeder for starch powder is entrained in the housing by the gas flow and deposited on a deposit, and foamed by increasing the temperature to the foaming temperature and simultaneously reducing the pressure in the housing. Further layers of starch powder can then be deposited and foaming repeated until a sandwich structure with the desired layer thickness results.

[0032] In order to be able to accurately meter the supply of starch powder, the supply device consists of a storage container connected to a lower angle tube which ends in the side wall of the gas inlet, in which there is furthermore a screw conveyor as a metering aid and supply device for the starch which is deposited on a deposit in the housing.

[0033] For this purpose, a gas flow in the gas inlet is used, which entrains the starch powder that reaches the gas inlet and deposits it on the site like "artificial snowfall".

[0034] After the desired layer thickness of the sandwich structure is reached, the gas inlet is stopped and the vacuum high temperature furnace is heated to 1,000° C. or 2,000° C. for carbonization or graphitization of the sandwich structure.

[0035] The present invention will now be described in more detail with reference to examples. [Brief description of the drawings]

[0036] [Figure 1] FIG. 1 is a schematic diagram of a continuous furnace suitable for carrying out the method. [Diagram 2] FIG. 1 is a schematic diagram of a vacuum high temperature furnace suitable for carrying out the method and for carbonization and graphitization. EXAMPLES

[0037] The apparatus for carrying out the method consists, according to FIG. 1, of a box 1 filled with argon or nitrogen, in which there is a furnace 2 equipped with a heating zone 3 for receiving a sheet-like substrate 4 coated with starch powder 4′.

[0038] The oven 2 can be a continuous oven that can be evacuated by means of a box 1, which is equipped with a conveying device in the form of a heat-resistant conveyor belt 5 or the like for conveying the sheet-like substrate 4 coated with starch powder from a feed position 6 to a heating zone 3 (foaming zone) and from there to a discharge position 7, where the feed position 6 and the discharge position 7 can be either in the same place or adjacent to each other in space.

[0039] Furthermore, above the feed position 6 there is a container 9 connected to a shaking device 8 , which contains the starch powder to be spread onto the sheet-like substrate 4 .

[0040] Furthermore, the container 9 is equipped with a sieving device 10 to ensure that only starch powder with the same particle size is spread onto the sheet base 4, as a prerequisite for uniform foaming in the heating zone 3. In principle, instead of starch powder, starch in granular form can also be used, i.e. with a slightly coarser particle size than the powder.

[0041] Also, instead of starch powder, other polymers or polymer mixtures can be used, provided that they melt at relatively high temperatures and simultaneously release gas.

[0042] In summary, furnace 2 should have the following characteristics: It must be possible to create a negative pressure by suction in box 1, and therefore also in oven 2, which in this case remains open. - Furnace 2 in box 1 should be able to operate with different gases provided by box 1. - Box 1 should be filled with argon, nitrogen, and should be able to be filled / purged with air. The furnace 2 should be able to reach an internal temperature of 450°C. - Box 1 / furnace 2 should be equipped with an exhaust system to prevent the gases generated in the furnace from coming into contact with the air and forming a flame.

[0043] For the production of microporous carbon foam sheet 11, in a first step at least one thin layer of foamable starch powder or at least one thin layer of other foamable polymer or polymer mixture in powder form is spread onto a heat-resistant sheet-like base 4. This is preferably done at feed position 7 inside box 1.

[0044] The sheet-like base 4 coated with starch powder is then transported into the oven 2 by means of a conveyor belt 5. Afterwards, air, argon or nitrogen is introduced into the oven 2 by replacing the air or pumping it out of the high-temperature oven until an internal pressure of 720 mbar is adjusted in the oven. It is important here that in the oven 2 it must be possible to adjust the internal pressure from about 720 mbar to about 1,000 mbar.

[0045] It is also important that the furnace 2 present in the box 1 is always open or is used without a door, so that the required internal pressure in the furnace 2 can be adjusted through the box 1, through which the required gases are also introduced into the furnace 2. In this way, no time is lost for gas and pressure adjustments, which allows for more efficient operation.

[0046] The furnace 2 is then heated to 180-450°C (foaming temperature) in the heating zone 3 in order to foam the starch. If air is introduced into the furnace 2 instead of argon or nitrogen, the foaming temperature of 250°C must not be exceeded.

[0047] Otherwise, the particular foaming temperature selected will depend on the internal pressure, the type of gas, the polymer being foamed, and the gas flow rate into the furnace. For example, a higher gas flow rate will require a higher foaming temperature due to the increased cooling effect.

[0048] In order to stabilize the expanded starch on the sheet substrate 4 into a expanded sheet 11, the expansion temperature in the heating zone 3 of the oven 2 must be maintained for an extended period of time, up to 10 hours.

[0049] To produce thicker foam sheets 11, it is sufficient to transport the sheet-like substrate 4 coated with starch powder back to the feed position 6 for the application of a further layer and then again into the oven 2 by means of the conveyor belt 4. This process can simply be repeated, the temperature in the oven being kept constant, until the desired thickness of the foam sheet 11 is achieved.

[0050] The foamed sheet 11, which has been made easier to handle by stabilization, is finally carbonized in another furnace at >1,000° C. in vacuum or in protective gas or graphitized at >2,000° C. Such a furnace can be a vacuum high-temperature furnace.

[0051] If the sheet-like substrate 4 consists of a graphitizable material such as a polymer, for example polyacrylonitrile, it can remain bonded to the foam sheet 11 during carbonization and possibly graphitization. In the simplest case, the sheet-like substrate can consist of paper. If the substrate consists of polyacrylonitrile, it can be stabilized.

[0052] Alternatively, the sheet base 4 can be made of graphite or graphite covered with paper or other heat-resistant material, in which case the stabilized foam sheet 11 can be peeled off from the sheet base 4 before carbonization, since in this case the sheet base 4 does not adhere to the stabilized foam sheet 11.

[0053] The carbonized foam sheet 11 may also be graphitized at temperatures >2,000° C. in a suitable furnace, for example in a separate vacuum high temperature furnace.

[0054] If a relatively thick foamed sheet 11 is to be produced, after each foaming, a further layer of foamable starch powder is sprinkled onto the already existing relatively thin foamed sheet, and foaming is repeated until a sandwich structure results.

[0055] In this case, however, carbonization or graphitization can only take place when all layers of the sandwich structure have been completely expanded.

[0056] The structured surface of the foamed sheet 11 can be achieved by applying starch powder onto the sheet-like substrate 4 in locally different layer thicknesses.

[0057] In the case of a relatively thickly applied layer, it is important and necessary to reduce the foaming temperature by at most 10% in order to facilitate the diffusion of the gas generated during foaming and to achieve uniform foaming and prevent the occurrence of coarse bubbles.However, for this purpose, the time during which the foaming temperature is kept constant must be correspondingly long.Alternatively, "fresh" starch can be periodically sprinkled on the foamed starch.

[0058] In principle, the entire "foaming" and "carbonization / graphitization" processes can be carried out in the same furnace, provided that this furnace is a closed vacuum high-temperature furnace capable of adjusting the temperatures required for each process step.

[0059] FIG. 2 shows a schematic diagram of a vacuum high temperature furnace 12 suitable for carrying out the method of producing carbon foam plate 11 from starch powder or other foamable polymer / polymer mixture in powder form as well as for carbonization and graphitization.

[0060] The vacuum high-temperature furnace 12 consists of a vacuum-tight housing 13, which is connected to a pump (not shown) for generating a negative pressure via a connection port 14. Furthermore, in the vacuum high-temperature furnace 12 there is a place 15 for placing the sheet-like base 4 for the foamed sheet 11 to be formed on the sheet-like base 4, for example in the form of a sandwich structure.

[0061] Furthermore, a gas inlet 16 is provided on the side of the upper region of the vacuum high-temperature furnace 12, which projects into the vacuum high-temperature furnace 12 at a space above the edge of the storage area 15.

[0062] This gas inlet 16 is connected on the one hand to a source, not shown, of inert gas 16' and via an angle tube 17 to a screw conveyor 18 and a storage container 19 for the starch 20 thereon.

[0063] The supply of powdered starch 20 into the high-temperature vacuum furnace 12 is carried out with the aid of inert gas flowing into the vacuum high-temperature furnace 12 via the gas inlet 16, which, as a result of the gas flow through the gas inlet 16, entrains the starch 20, which is conveyed from the storage container 18 to the angle tube 17 using the screw conveyor 18, and deposits it on the sheet-like base 4 present on the deposit 15, like covering the ground with artificial snow, and causes it to foam when the foaming temperature is reached.

[0064] In this way it is possible to produce thick foam sheets 11. Alternatively, the temperature can be alternately increased and decreased to obtain more or less dense layers.

[0065] A sandwich structure can also be produced by first depositing a layer of starch 20, which is then expanded, and then depositing and expanding yet another layer of starch 20, and so on.

[0066] Once the desired layer thickness of the foam sheet 11 is reached, the gas supply can be stopped and the vacuum high temperature furnace can be heated to 1,000° C. or 2,000° C. for carbonization or graphitization. [Explanation of symbols]

[0067] 1 Box 2 furnace 3 Heating Zones 4 Sheet base 4' Starch powder 5 Conveyor belt / transport device 6 Supply position 7 Ejection position 8 Shaking device 9 containers 10 Sieve device 11 Foam Sheet 12 Vacuum high temperature furnace 13. Housing 14 Connection Ports 15 Storage 16 Gas inlet 16' Inert Gas 17 Angle pipe 18 Screw conveyor 19 Storage Containers 20. Starch

Claims

1. - applying at least one layer of foamable starch powder as thin as possible onto a heat-resistant sheet base, - placing said base coated with starch powder in an oven; - introducing argon or nitrogen into the furnace by replacing the air or by pumping the air out of the furnace until an internal pressure of about 720 mbar to about 1,000 mbar is adjusted; - Heating the oven to a foaming temperature of 180°C up to 450°C to foam the starch powder on the base; and - maintaining the foaming temperature for a relatively long period of time, up to 10 hours, to stabilize the foamed starch into a foamed sheet; A method for producing carbon foam sheets from starch powder or other foamable polymer / polymer mixtures in powder form by:

2. 2. The method according to claim 1, characterized in that the foam sheet 11, which has been stabilised and made handleable, is carbonised at >1,000° C. or graphitised at >2,000° C. in a vacuum or in a protective gas.

3. 2. The method according to claim 1, characterized in that as foamable starch powder corn starch, tapioca starch, wheat starch or starch powder obtained on the basis of oats or cereals is used.

4. 2. The method of claim 1, wherein the heating to the foaming temperature is performed at a heating rate of about 5° C. / min.

5. The method according to any one of claims 1 to 4, characterized in that the sheet-like substrate is made of a graphitizable material.

6. Method according to any one of claims 1 to 4, characterized in that the sheet-like base is made of steel, graphite or other heat-resistant material.

7. 7. The method according to any one of claims 1 to 6, characterized in that the expandable starch powder is sieved before or during its spreading on the sheet-like substrate.

8. 8. The method according to claim 1, characterized in that after each foaming of the starch powder, a further layer of foamable starch, as thin as possible, is applied onto the already existing relatively thin foamed foam sheet and foaming is repeated.

9. 9. The method according to claim 1 , characterized in that the starch powder is applied to a sheet-like base or to an already foamed foam sheet in locally different layer thicknesses in order to form a structured surface of the foam sheet.

10. 10. The method according to claim 1, characterized in that the foaming temperature is reduced by about 10% when a relatively thick layer of starch powder is applied.

11. 11. The method according to claim 1, wherein as sheet-like substrate a graphitizable material, such as a polymer, for example polyacrylonitrile, is used.

12. 1. An apparatus for carrying out a method for producing carbon foam sheets from starch powder or other foamable polymer / polymer mixtures in powder form, characterized in that an oven (2) is arranged in a box (1) filled with argon or nitrogen, said oven (2) comprising a heating zone (3) for placing a sheet-like substrate (4) coated with starch, said oven (2) being a continuous oven equipped with a conveying device (5) for conveying the sheet-like substrate (4) coated with starch from a feed position (6) to the heating zone (3) and from there to a discharge position (7), and that the feed position (6) and the discharge position (7) can be in the same place or next to each other in space.

13. 13. Apparatus according to claim 12, characterized in that the conveying device (5) is a high-temperature resistant conveyor belt.

14. 14. Apparatus according to claim 12 or 13, characterized in that above the feed position (6) there is arranged a container (9) for containing starch, connected to the shaking device (8), and that the container (9) is equipped with a sieve device (10).

15. 1. An apparatus for carrying out a method for producing carbon foam sheets from starch powder or other foamable polymer / polymer mixtures in powder form, characterized in that a heatable vacuum high-temperature oven (12) is equipped with an evacuable, insulated housing (13) which is provided with a gas inlet (16) for an inert gas, protruding into the housing (13) from a lateral side, said gas inlet (16) being simultaneously connected to a supply device for starch powder (20) and in which a place (15) for depositing the starch powder (20) in layers is present below the gas inlet (16).

16. 16. Apparatus according to claim 15, characterized in that the feeding device consists of a storage vessel (19) connected to an angle tube (17) below it, which ends in the side wall of the gas inlet (16), and in which a screw conveyor (18) is present in the angle tube (17) as a metering aid for the starch (20) to be deposited on a deposit (15) in the housing (13) by means of a gas flow of the supplied inert gas (16') or air formed in the gas inlet (16).

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